The invention is based on a method for synchronizing one or more receivers to a transmitter within a transmission system, using a data stream with guard intervals, in particular for compensating for multi-path propagation, and on a transmitter for preparing a synchronization train and a receiver for evaluating this synchronization train, and on a communications system.
It is assumed for instance that one transmitter serves one or more receivers. The transmitters sends one or more packets to the receivers at the same instant.
In a transmission system, which in particular uses OFDM (Orthogonal Frequency Division Multiplexing), the problem of synchronization arises. In OFDM transmission, the transmission symbols are modulated over a plurality of subcarriers in the frequency range by a generally digital type of modulation [1]. The subcarriers are then transformed into the time range together by an IFFT (Inverse Fast Fourier Transformation) and then transmitted.
In the receiver, it is necessary to reconstruct some information by way of the signal transmitted, especially to reconstruct the beginning of the block and the center frequency error.
To ascertain the beginning of the block, the chronological position of the signal to be received must be known. For this purpose, a two-stage method is usually used, according to which first a coarse and then a fine detection of the beginning of the block are performed in succession.
In the normal situation, the receiver has a center frequency error compared to the sensor. In the case of OFDM, this error is especially critical, because it can cause a disruption of orthogonality, which leads to increased bit errors. The frequency synchronization serves to correct this frequency difference.
In a communications system with transmission of relatively short data packets, to achieve a synchronization, two identical synchronization symbols, especially OFDM symbols, are made to precede a transmission burst in accordance with [2], [3] and [4], and these are transmitted twice with a predetermined spacing. The position of these signals can be determined by evaluation of the metrics.
To avoid intersymbol interferences (ISI), a guard interval is often inserted into the transmitter in conjunction with the OFDM transmission technique, the length of which guard interval is adapted to the duration of the channel pulse response. In order for there not to be any actual disruption in the receiver from chronologically adjacent symbols, the instant of synchronization, that is, the onset of the ISI-free signal portion, is ascertained before the data evaluation in the receiver. The ascertainment of this instant is called block or symbol synchronization. When the pulse response of the present channel is shorter than the guard interval, the block synchronization need not ascertain the beginning of the ON state precisely; instead, the result is an allowable synchronization interval.